Star and airplane trails using intervelometer and StarStax

I spent xmas break in California where the weather was clear and not too cold, so I took the chance to try some star trail photography for the first time. The chdk was critical - I used the ultra intervelometer script to take 15 second exposures over and over again (and to turn off the time-consuming dark frame subtraction). I usually left it on for as long as the battery would last, which was about 1-1.5 hours. Sadly, I forgot to bring my AC adapter, but I plan to try that at some point in the future to see if I can keep it going longer.

I did not try to actually extend the exposure time, rather I focused on stacking. I wonder if I should try for single long exposures next time?

The stacking worked great for stars, but the main downside was for airplanes, since they move faster you get very obvious gaps in the trails. I have started using Photoshop Elements to manually fill in the gaps in the airplane trails, but its very tedious. Here is an example of progress so far:

ETA: I also used Aperture to adjust the levels of the photos. On the camera's screen I could see stars, but when I put them on my computer they looked all dark until I turned up the exposure. I also turned up the blue saturation, and a few things like that. Aperture makes it easy to apply those same adjustments to tons of photos at once, so I did that and then exported them to StarStax.

« Last Edit: 29 / January / 2014, 12:53:44 by CanonBlue »

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My Flickr PageI use the chdk on my SX230 and SD1100 cameras, and I installed them using a Macbook, currently running 10.6.8.

I did not try to actually extend the exposure time, rather I focused on stacking. I wonder if I should try for single long exposures next time?

I wouldn't suggest trying to do very long shots single exposure, but you could go over 15 sec. If each shot was a couple minutes, the airplanes would only take a few frames at most.

Longer exposures will have more noise and "amp glow" (usually purple glow in the corners). Taking dark frames and subtracting them later can help with this, especially if you use raw. OTOH, raw save slower, so there will be bigger gaps between exposures. The sky will also over expose pretty quickly unless you are in a vary dark location.

I think I should definitely increase the exposure time, though I'm not sure exactly how much. The 15 sec exposures came out very very dark on the computer - I had to turn the exposure levels all the way up in Aperture to see the stars and the silhouettes of trees. So there is definitely room for increasing exposure time without getting too much glow from ambient light, but I guess I'll just have to play with it to see how much I want to increase it. Maybe a few minutes at least? That would also help with the airplanes, as you said. Unfortunately with the weather here in Chicago its likely to be quite awhile before I can try this again. The ones I posted so far were all taken over break in California, which was usually dry and clear even for there this winter.

Regarding dark frames, I did take a dark frame picture by hand and I put it into StarStax and tried to get it to subtract it, but it wouldn't work and I'm not sure how else to do it. With the 15 sec exposures there were only a couple hot pixels that were noticeable, so I ended up just removing them in Aperture by hand in the final photo, using the repair brush (I may have done that after I posted them to flickr, when I decided to make prints, so I'm not sure if those versions still have the hot pixels or not. Either way you really have to look closely to notice them.).

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My Flickr PageI use the chdk on my SX230 and SD1100 cameras, and I installed them using a Macbook, currently running 10.6.8.